1. Technical Field
The present invention relates to a radio wave absorption material formed of a sintered ferrite material for absorbing unnecessary radio waves and to a radio wave absorber formed of the radio wave absorption material.
2. Description of Related Art
With the development of communication technology and radio guidance technology for airplanes, etc. or with the proliferation of various electric/electronic appliances, such as television sets, radios and mobile phones, for use in prevention of malfunctions in controllers, for use in noise filters for various electric/electronic appliances, for use in radio wave absorption walls or for use in radio wave anechoic chambers, radio wave absorbers matching with the respective frequency bands are demanded for the purpose of shutting off high-frequency noise that may cause unnecessary signals.
As conventional radio wave absorbers, those formed of a metal plate, thin film, carbon, etc. having conductivity and being held with rubber, plastic, etc. are generally used. In addition, an absorber made of carbonyl iron known as a magnetic absorber, an absorber combined with rubber, etc., and an absorber made of styrofoam, etc. through the use of dielectric loss are also known. Furthermore, absorbers made of composite materials obtained by combining ferrite powders, such as Ni—Zn-based and Ni—Cu—Zn-based ferrite powders, with high molecular compounds, such as rubber, silicone resin and epoxy resin, absorbers made of plastic in which metal foil pieces, such as aluminum foil pieces, are mixed, absorbers made of conductive fiber, etc. have been proposed.
In recent years, a configuration having ferrite sintered body that uses the magnetic resonance phenomenon, that is, a Ni—Cu—Zn-based sintered ferrite plate lined with a metal plate, is excellent in radio wave absorption capability and can be used as a relatively thin plate, thereby being brought into actual use. Furthermore, composite materials obtained by combining or laminating the above-mentioned various materials have also been proposed.
Such conventional radio wave absorption materials have inherent matching frequency bands; for example, a radio wave anechoic chamber for use in various experiments and measurements is required not to allow radio waves generated inside the chamber to reflect from the walls thereof and is required to shut off radio waves coming from the outside, and materials excellent in radio wave absorption capability in wide frequency bands are demanded.
Hence, various technologies have been proposed in which Ni—Cu—Zn-based ferrite sintered materials having low reflection coefficients in relatively wide bands, being able to be made into thin sheets or small pieces and being excellent in mass productivity are used as radio wave absorption materials ideally suited for various electric/electronic appliances or radio wave anechoic chambers (refer to Japanese Patent No. 2794293, Japanese Patent No. 2898343, Japanese Patent No. 3178885, Japanese Patent No. 3278373, and Japanese Patent No. 3422709).
Since NiO being expensive among main materials for use in ferrite is used in such Ni—Cu—Zn-based ferrite sintered materials, there occurs a problem that Ni—Cu—Zn-based ferrite tiles produced as radio wave absorbers are high in cost.
Hence, Mn—Zn-based ferrite sintered materials serving as radio wave absorption materials being equivalent to the Ni—Cu—Zn-based ferrite sintered materials ill performance without using such a costly main material as NiO have been proposed (refer to Japanese Patent Application Laid-Open No. 2004-247602 and Japanese Patent Application Laid-Open No. 2004-247603).